1. Field of the Invention
The present invention relates to a three-panel projection display apparatus, and more particularly to a projection display apparatus for displaying a color image by separating light emitted from a light source into lights of three primaries, i.e., red, green, and blue, with color separating optical systems, by irradiating three modulating means with the separated lights, by combining the lights modulated by the respective modulating means with a color combining optical system, and by projecting the combined light through a single projection lens onto a screen.
2. Description of the Related Art
The optical systems of three-panel projection display apparatus, as they are applied to liquid crystal projectors, will be described below.
FIG. 1 of the accompanying drawings shows a conventional liquid crystal projector for projecting a color image onto a screen. As shown in FIG. 1, light emitted from light source 1 is reflected by reflecting mirror 2 and directed, as illuminating light having a uniform luminance distribution through two integra-tor lenses 4, 5 and first collective lens 7, toward a liquid crystal light valve that serves as a modulating means. Before the light is applied to first collective lens 7, it is converted into S-polarized light by polarization converter 6 according to polarization separation and polarization conversion.
White light that has passed through first collective lens 7 is separated into blue light B and green red light G-R by dichroic mirror 8 that serves as a first color separating optical system for reflecting blue light and passing red and green lights. The green red light G·R that has passed through dichroic mirror 8 is separated into green light G and red light R by second dichroic mirror 9 that serves as a second color separating optical system for reflecting green and blue lights and transmitting red light.
The blue right B separated by dichroic mirror 8 is reflected by reflecting mirror 10 and passes through condenser lens 13A to liquid crystal light valve 15A. The green light G separated by dichroic mirror 9 passes through condenser lens 13B to liquid crystal light valve 15B. The red light R separated by dichroic mirror 9 is applied to liquid crystal light valve 15C by a relay optical system comprising two relay lenses 17, 18 and two reflecting lenses 11, 12, and condenser lens 13.
Liquid crystal valves 15A, 15B, 15C, which correspond to the blue, green, and red lights, respectively, are combined with polarizing panels 14A, 14B, 14C that are positioned on the entrance sides of liquid crystal valves 15A, 15B, 15C, respectively, and polarizing panels 16A, 16B, 16C that are positioned on the exit sides of liquid crystal valves 15A, 15B, 15C, respectively. These polarizing plates serve to align the planes of polarization of the polarized lights that are modulated by the liquid crystal valves. The polarizing panels on the entrance sides and the polarizing panels on the exit sides are angularly arranged such that their transmission axes extend perpendicularly to each other. Only lights that are polarized in a direction parallel to the transmission axis of the polarizing panels on the exit sides pass through the polarizing panels on the exit sides, and lights that are polarized in other directions are absorbed by the polarizing panels on the exit sides. The lights that are applied to liquid crystal valves 15A, 15B, 15C are modulated thereby, and the modulated lights are combined with each other by cross dichroic prism 19 that serves as a color combining optical system. The combined light is then projected at an enlarged scale onto projection screen 21 by projection lens 20.
Condenser lenses 13A, 13B that are disposed respectively in blue and green light paths, which are of the same length, are identical to each other. Distance b between condenser lens 13A in the blue light path and liquid crystal light valve 15A is the same as distance a between condenser lens 13B in the green light path and liquid crystal light valve 15B.
FIG. 2 of the accompanying drawings shows another conventional liquid crystal projector having an optical system wherein second collective lens 31A is disposed in the blue light path between dichroic mirror 8 and reflecting mirror 10 and second collective lens 31B is disposed in the green light path between two dichroic mirrors 8, 9. Second collective lenses 31A, 31B that are disposed respectively in the blue and green light paths, which are of the same length, are identical to each other. Second collective lenses 31A, 31B are positioned at the same distance respectively from liquid crystal light valves 15A, 15B in the respective light paths. Specifically, the light path length (e+f) from liquid crystal light valve 15A via reflecting mirror 10 to second collective lens 31A in the blue light path is the same as the light path length (c+d) from liquid crystal light valve 15B via dichroic mirror 9 to second collective lens 31B in the green light path.
In the liquid crystal projectors described above, the effective pixel regions of the liquid crystal light valves are uniformly illuminated by an integrator illuminating system that comprises two integrator lenses (fly-eye lenses) 4, 5 and collective lens 7. There is also known a projection display apparatus incorporating a rod lens illuminating system, rather than an integrator illuminating system, which comprises a rod lens and two collective lenses for illuminating liquid crystal light valves with lights having a uniform luminance distribution.
With the cross dichroic prism being used as described above, only one of the distances (light path lengths) from the collective lenses to the liquid crystal light valves in the respective light paths is longer than the other distances. In the liquid crystal projectors shown in FIGS. 1 and 2, the distances from the collective lenses to the liquid crystal light valves in the blue and green light paths are the same as each other, but the distance from the collective lens to the liquid crystal light valve in the red light path is longer than the corresponding distances in the blue and green light paths. Therefore, the amount of light applied to the liquid crystal light valve in the red light path is smaller than the amounts of light applied to the liquid crystal light valves in the blue and green light paths. The relay optical system that include relay lenses 17, 18 is disposed in the red light path for the purpose of uniformizing the amounts of light applied to the respective liquid crystal light valves in the red, blue, and green light paths. The type of the projection display apparatus which employs the relay optical system in the red light path is referred to as a red relay type, and the three-panel liquid crystal projectors shown in FIGS. 1 and 2 are of the red relay type.
If dichroic mirror 8 shown in FIGS. 1 and 2 is replaced with a component for reflecting red light and passing green and blue lights and dichroic mirror 9 is replaced with a component for reflecting green and red lights and transmitting blue light, then the distance from the collective lens to the liquid crystal light valve in the blue light path is longer than the distances from the collective lenses to the liquid crystal light valves in the red and green light paths. In this arrangement, the relay optical system is employed in the blue light path, and this type is referred to as a blue relay type.
In the projection display apparatus such as the liquid crystal projectors described above, the liquid crystal light valves have respective image forming areas smaller than the areas thereof that are illuminated by the light emitted from the light source. The image forming areas that are smaller than the illuminated areas are prevented from protruding out of the illuminated areas even if the illuminated areas are vertically or horizontally shifted due to errors with respect to the positioning accuracy and focal lengths of the integrator lenses. Such an area setting allows the image forming areas of the liquid crystal light valves to be accurately illuminated by the light emitted from the light source.
However, if the illuminated areas are too large compared with the image forming areas, then the image projected onto the projection screen will not have sufficient brightness. If the illuminated areas are of the same size as the image forming areas, then when the components of the integrator illuminating system suffer an error, the illuminated area tends to be shifted out of alignment with the image forming areas, possibly producing a shaded region on an edge of the projected image.
One solution to the above problems is provided by a process disclosed in Japanese laid-open patent publication No. 115799/1998.
According to the process disclosed in Japanese laid-open patent publication No. 115799/1998, the positions of integrator lenses, the position of the first collective lens, and the angles of the reflecting mirrors can be fine-adjusted for adjusting the illuminated areas in the respective light paths out of misalignment. The disclosed process makes it unnecessary to set wide margins around the image forming areas of the light crystal light valves in view of possible displacements of the illuminated areas. As any margins to be set around the image forming areas may be very small, the illuminating light can be utilized efficiently and the brightness of projected images is increased. Even though the margins around the image forming areas are small, since the positions of integrator lenses, the position of the first collective lens, and the angles of the reflecting mirrors can be fine-adjusted, the image forming areas will not protrude partly out of the illuminated areas and no shaded region will be formed on an edge of the projected image.
According to the process disclosed in Japanese laid-open patent publication No. 115799/1998, however, the illuminated areas in the respective light paths are adjusted out of misalignment simply by fine-adjusting the positions of integrator lenses, the position of the first collective lens, and the angles of the reflecting mirrors. Even if the process disclosed in Japanese laid-open patent publication No. 115799/1998 is applied to the red relay type shown in FIGS. 1 and 2 where the distance from the collective lens to the liquid crystal light valve in the red light path is longer than the corresponding distances in the blue and green light paths, the illuminated area in the blue light path is not clearly defined and is smaller than the illuminated area in the green light path. If the liquid crystal projector is designed in accordance with the illuminated area in the blue light path, then the illuminated area in the green light path will become larger than necessary, resulting in a reduction in the brightness of the projected image.
The reasons for the above problem are as follows: As regards the red relay type shown in FIGS. 1 and 2, since the refractive indexes of glass materials of the polarizing plates with respect to the wavelengths of lights used differ widely from each other in the green and blue light paths whose light path lengths up to the light valves are equal to each other, even if the illuminated area in the green light path have sharply defined edges, the illuminated area in the blue light path is blurred due to a different axial chromatic aberration, resulting in a reduced effective illuminated area.
Regarding the blue relay type, since the refractive indexes of glass materials of the polarizing plates with respect to the wavelengths of lights used are close to each other in the red and green light paths whose light path lengths up to the light valves are equal to each other, there is no large difference between the distributions of the amounts of light on the illuminated areas in the red and green light paths. A relay optical system having different light path lengths is free from the problems of the red relay type because it is possible to increase or to reduce design values of the illuminated areas based on the layout of lenses, and also to reduce aberrations.